When you add a shard to a sharded cluster, you affect the balance of
chunks among the shards of a cluster for all existing sharded
collections. The balancer will begin migrating chunks so that the
cluster will achieve balance. See Cluster Balancer for
more information.

Changed in version 2.6: Chunk migrations can have an impact on disk space. Starting in
MongoDB 2.6, the source shard automatically archives the migrated
documents by default. For details, see moveChunk directory.

Add a shard to the cluster using the sh.addShard()
method, as shown in the examples below. Issue sh.addShard()
separately for each shard. If the shard is a replica set, specify the
name of the replica set and specify a member of the set. In
production deployments, all shards should be replica sets.

Optional

You can instead use the addShard database
command, which lets you specify a name and maximum size for the
shard. If you do not specify these, MongoDB automatically assigns
a name and maximum size. To use the database command, see
addShard.